Summary of fifteen years of HVDC experiments and tests in vacuum at the High Voltage Padova Test Facilities

This paper summarizes the main results obtained in the last 15 years at the High Voltage Padova Test Facilities dedicated to support the design and the commissioning of the ITER Neutral Beam injector.Several experiments and tests in High Voltage Direct Current were performed in high and medium vacuum on electrodes separated by gaps, from a few mm up to 150 mm, and with voltages up to 800 kVdc. Different types of surface finishing, electrodic configurations, surface treatments and coatings were tested with the aim to gain knowledge on the several aspects influencing the de voltage holding in high vacuum.Systematic tests were performed in high vacuum on stainless steel electrodes to validate a probabilistic model to predict the maximum voltage of a generic-shape, multielectrode systems isolated by high vacuum gaps. Such tests have confirmed the existence of a marked “total voltage effect” from gap lengths larger than few centimeters. Tests in Ar, H2 and N2, were carried out to characterize the “pressure effect” corresponding to the voltage holding increase observable in medium vacuum until the left side of the Paschen curve is reached. Furthermore, the effect of the application of a static magnetic field on the Townsend discharge and how the Paschen curve is consequently modified was clarified experimentally. Endurance tests have been performed by applying stationary electric fields, up to $60 \mathrm{kV} / \mathrm{mm}$, for several days on stainless steel electrodes. These tests made it possible to observe the irreversible switch-on phenomena with currents of tens of microA on electrodes separated by $\mathbf{1 ~ m m}$ gaps.Experimental evidences regarding the spatial distribution of the power dissipated during micro-discharges in high vacuum, have been explained by the existence of charged particles accumulation points in case of electrodes separated by large vacuum gaps. A mutual exchange of charged particles between electrodes seems a reasonable physical mechanism to interpret the experimental observations. An intense development activity on the X-ray detectors has been carried out. A spectral and intensity measurement system of the X rays emission produced by the bremsstrahlung radiation has been developed which allows continuously monitoring the state of the electrode conditioning without interfering with the test in progress and without inserting any device in vacuum. The analysis of X-rays emitted on time scales of the order of microseconds has made it possible to identify, in a large number of the observed vacuum discharges ($\approx \mathbf{4 0 \%}$), that bursts of electrons impinging the anodic surface are the breakdown precursors. The consequent Electron Stimulated Desorption of gas from the anodic surface have been considered a key player in the development of the electrical discharge and in the collapse of the voltage between the electrodes. Finally, tests have been carried out on solid insulators, optical fibers and spark gaps. A review concerning the lessons learned, the open issues and the next activities is also presented.